Reversible Electron Transfer Reactions Research Articles

Reversible Electron Transfer Reaction between Polyaniline and Thiol/Disulfide Couples

Reversible electron transfer was observed between polyaniline (PAn) and thiol/disulfide couples of 2,5-dimercapto-1,3,4-thiadiazole (DMcT), 2-mercapto-5-methyl-1,3,4-thiadiazole, 2-mercaptopyridine, and thiophenol. Thus, PAn can be used as a molecular current collector for those insulating organosulfur compounds, which are promising high-capacity energy storage materials. Among those couples, DMcT exhibits the fastest reversible electron transfer. Electron transfer from other aromatic and aliphatic thiols to oxidized PAn is also observed. Effects of protons on the reactions and reaction kinetics are discussed.

Mechanistic study of the reversible electron transfer reactions between horse heart cytochrome C and a series of cobalt phenanthroline complexes

Mechanistic study of the reversible electron transfer reactions between horse heart cytochrome C and a series of cobalt phenanthroline complexes

Dependence of electron transfer kinetics of the ferrocene/ferrocenium couple on the viscosity in ambient temperature chloroaluminate ionic liquids

The heterogeneous electron transfer kinetics of the ferrocenium/ferrocene couple have been measured throughout the basic range of composition of the ambient temperature chloroaluminate ionic liquid composed of 1-ethyl-3-methylimidazolium chloride (ImCl) and aluminum chloride. Normal and reverse pulse voltammetries with pulse times as short as 5 μs were employed. After subtraction of the background currents, the experimental voltammograms were directly compared with those calculated from the COOL algorithm using a model for reversible and quasi-reversible electron transfer reactions. The standard rate constants, uncorrected for double layer effects, were found to decrease from 0.45±0.35 cm s −1 at 0.95:1 to 0.027±0.004 cm s −1 at 0.45:1 mole ratio of aluminum chloride to ImCl. Possible factors contributing to the observed dependence are discussed. The most likely reason for the retardation of the electron transfer rate is the slow relaxation of the increasingly associated basic ionic liquid.

Polymer Solvents For Electrochemical Reactions

Solubility of inorganic salts and potential window of poly(ethylene oxide) (PEO) were analyzed. Sufficient potential window (−1.6 to + 1.6 V vs Ag) was obtained for salt-containing PEOs when the ionic conductivity of the PEOs was higher than 3.0 × 10−4 S/cm. PEO was then used as a polymer solvent for electrochemical redox reactions of heme proteins. Myoglobin was solubilized in salt-containing PEO oligomers only after PEO modification, and their reversible redox reactions were confirmed. The electrochemical reduction was slow because of the very low diffusion coefficient of the proteins in PEO oligomers. PEO-modi-fied myoglobin and hemoglobin showed reversible electron transfer reaction with ITO glass electrode at even 80 or 100°C in PEO oligomers.

An integral equation approximation for the dynamics of reversible electron-transfer reactions

The solution to an integral equation [J. Zhu and J. C. Rasaiah, J. Chem. Phys. 96, 1435 (1992)] for the survival probabilities in the Sumi–Marcus model of reversible electron-transfer (ET) reactions, in which ligand vibrations and fluctuations in the solvent polarization play important roles, is obtained numerically using a simple computer program suitable for use on a PC. The solutions depend on the time correlation function Δ(t) of the reacting intermediates along the reaction coordinate which is shown to be equal to the time correlation function of the Born free energy of solvation of these intermediates even in discrete molecular solvents provided its response is linear. This enables Δ(t) to be determined accurately from time-delayed fluorescence Stokes shift experiments or from dynamical theories of ion solvation; it is usually an exponential (Debye solvent) function of time or a sum of such exponentials (non-Debye solvent). The solutions to the integral equation, which can be obtained numerically for any given Δ(t), are found to predict the electron-transfer dynamics successfully over a wide range of model parameters. They can also be approximated by single or multiexponential interpolation formulas in which the thermally equilibrated rate constants are modified by a factor which reflects the relative importance of ligand (or inner-sphere solvent) vibration and outer-sphere solvation dynamics. The use of an effective longitudinal relaxation time in calculations of ET rates in solution is shown to be a poor assumption in some solvents. The theory is compared with an experiment in the inversion region, and its extension to include high-frequency vibrational modes that lead to an increased ET rate in other experiments is discussed.

Charge separation in donor-chromophore-acceptor complexes: inverted region behavior in reverse electron transfer reactions

Time-resolved absorption studies which elucidate intramolecular electron transfer rates have been performed on a series of covalently linked Ru(bipyridine) 3 -donor-acceptor complexes. In these complexes the electron donor is a phenothiazine moiety linked to a bipyridin by a (-CH 2 -) 4 chain, and the electron acceptor is an N,N'-diquaternary-2,2'-bipyridinium moiety, linked to a bipyridine by a (-CH 2 -) 2 chain

Reversible electron transfer dynamics in non-Debye solvents

The general solutions obtained earlier [J. Chem. Phys. 95, 3325 (1991)] for the coupled diffusion-reaction equations describing reversible electron transfer reactions in Debye solvents, governed by Sumi–Marcus free energy surfaces, are extended to non-Debye solvents. These solutions, which depend on the time correlation function of the reaction coordinate Δ(t), are exact in the narrow and wide window limits for Debye and non-Debye solvents and also in the slow reaction and non-diffusion limits for Debye solvents. The general solution also predicts the behavior between these limits and can be obtained as the solution to an integral equation. An iterative method of solving this equation using an effective relaxation time is discussed. The relationship between Δ(t) and the time correlation function S(t) of Born solvation energy of the reacting intermediates is elucidated.

Network Simulation of a Reversible Electron Transfer under Cyclic Voltammetric Conditions

Reversible electron transfer reaction of a freely diffusing species has been studied using the network approach. A network model for this process under cyclic voltammetric conditions has been proposed, and a numerical simulation has been made with the help of the electrical circuit simulation routine PSPICE. A great accuracy in simulated voltammograms has been reached with a personal computer at a moderate cost in computer time.

Dynamics of reversible electron transfer reactions

The dynamics of reversible electron transfer reactions in Debye solvents are studied by employing two coupled diffusion–reaction equations with the rate constants depending on the reaction coordinate. The equations are solved analytically in four limiting cases: fast and slow reactions as well as wide and narrow reaction windows. A general solution for the survival probabilities is obtained by employing a decoupling approximation similar to the one used by Sumi and Marcus [J. Chem. Phys. 84, 4896 (1986)] for nonreversible reactions; our solution verifies the existence of four limiting cases and also predicts the behavior between these limits. Interpolation between long and short time approximations to the general solution, leads to survival probabilities with a single exponential time dependence and rate constants ki satisfying the relation k1/k2=exp(−βΔG0), where ΔG0 is the standard free energy change for the reaction. Multiexponential behavior of the survival probabilities is exhibited when higher order terms are included in the evaluation of the general solution, but this deteriorates to a single exponential, governed by a first order rate constant, at long times. In the narrow reaction window limit the multiexponential solution is exact when both the forward and reverse reactions are barrierless, and the behavior at long times is determined by a rate constant k=0.83 τ−1L where τL is the longitudinal relaxation time. Similar behavior is found when the forward reaction alone is barrierless and the barrier for the reverse reaction is large (βΔG*1=0, βΔG*2≫1), except that the forward rate constant k1≊τ−1L [0.6+(π/βΔG*2)1/2]−1 depends on the barrier height for the reverse reaction which has a small rate constant. Our solutions reduce to those of Sumi and Marcus when the reverse reaction is ignored. They are also compared with numerical solutions to the diffusion reaction equations. The extension to non-Debye solvents is briefly discussed.

The Anion Radicals of Pheophytin a and its Derivatives Studied by Means of CIDNP Technique

Abstract We applied the Chemically Induced Dynamic Nuclear Polarization (CIDNP) technique to the photochemical reaction of pheophytin a (P1) and 4-methoxyphenol (1) in benzene-d6. The observed CIDNP effects were explained in terms of reversible electron-transfer reactions from 1 to 3(P1)* and from (P1)\ewdot to (1)\underset.+. The polarized signals of P1 were induced from the in-cage recombination of the triplet radical ion pair.

Reversible electron-transfer properties of the titanium(II) organometallic species (Cp)2Ti(bpy)

The electrochemical and spectroelectrochemical properties of (Cp){sub 2}Ti(bpy) in tetrahydrofuran (THF) with tetrabutylammonium perchlorate (TBAP) as supporting electrolyte are presented. This formal Ti(II) species exhibits four reversible electron-transfer reactions on the cyclic voltammetric time scale.

PHOTO‐CIDNP EVIDENCE FOR CHARGE‐TRANSFER INTERACTION BETWEEN SYNTHETIC WATER‐SOLUBLE PORPHYRINS AND GUANINE*

Abstract Photoreactions of aqueous solutions of synthetic water‐soluble porphyrins were studied by the 1H and 13C CIDNP technique. Strong polarizations, which were very sensitive to the presence of added acid, were observed on the cationic porphyrins (TMePyPH2‐TAPPH2) when irradiated through continuous UV‐visible light. They resulted from the reverse electron transfer between the semi‐oxidized and the semi‐reduced species of the derivative.When the experiments were carried out in the presence of nucleobases, guanine (and its derivatives) was the only residue that was polarized. This is thoroughly interpreted in terms of a reversible electron transfer reaction leading to guanine photooxidation by the porphyrin excited triplet state. It was shown to be drastically pH‐dependent and was correlated to the redox potential of the porphyrin. It was not affected by the incident wavelength. The reaction proceeded through the intermediate formation of the correlated radical‐ion pair: porphyrin radical anion‐guanine radical cation. This study suggested that a Type I (free radical) reaction could be one of the primary processes in DNA photosensitization by porphyrins.

ChemInform Abstract: FT ESR Study of Photoinduced Electron Transfer

With Fourier transform ESR (FT ESR) well resolved spectra of organic free radicals can be obtained even if the free induction decay, following a ..pi../2 microwave pulse, is as short as 1 ..mu..s. This makes the technique particularly suitable for the study of short-lived radicals. Results are presented of a FT ESR study of a transient free radical generated in a reversible photoinduced electron transfer reaction. The reaction and spin dynamics have been monitored from the time the radical is generated with nanosecond time resolution. The technique provides data complementary to those provided by flash photolysis.

FT ESR study of photoinduced electron transfer

With Fourier transform ESR (FT ESR) well resolved spectra of organic free radicals can be obtained even if the free induction decay, following a ..pi../2 microwave pulse, is as short as 1 ..mu..s. This makes the technique particularly suitable for the study of short-lived radicals. Results are presented of a FT ESR study of a transient free radical generated in a reversible photoinduced electron transfer reaction. The reaction and spin dynamics have been monitored from the time the radical is generated with nanosecond time resolution. The technique provides data complementary to those provided by flash photolysis.

Square wave voltammetry at small disk electrodes: Theory and experiment

An equation is derived for the current response of a reversible electron transfer reaction for square wave voltammetry at an embedded disk electrode. Peak shape and position are invariant to the dimensionless parameter Dτ/ r 2 where D is the diffusion coefficient, τ the square wave period, and r the radius of the disk, whereas peak current density increases without limit with increasing Dτ/ r 2. A simple empirical equation predicts the peak current for any value of Dτ/ r 2 for square wave amplitude 50/ n mV and step height 10/ n mV. Experimental results for oxidation of ferrocyanide at small platinum electrodes agree well with the theory and demonstrate the practical utility of the experiment.

CHLOROPHYLL PHOTOSENSITIZED ELECTRON TRANSFER IN PHOSPHOLIPID BILAYER VESICLE SYSTEMS: CORRELATIONS BETWEEN KINETIC PARAMETERS AND SOLUBILITIES OF VIOLOGEN ACCEPTORS *

Abstract— Twelve viologens (quaternary 4,4′‐bipyridinium ions) are investigated as electron acceptors in vesicle suspensions containing chlorophyll‐a (Chl) using laser flash photolysis. The structures of the viologens, which determine how they and their reduced radicals partition between the water and bilayer membrane phases, are systematically varied in order to probe the effects of acceptor solubility on the kinetics of photosensitized electron transfer reactions. The effectiveness of the viologens as quenchers of the Chl triplet excited state increases as they become less soluble in water, because the viologens are more likely to be incorporated into the membrane, but the efficiency of radical formation decreases and the rate of the reverse electron transfer reaction increases. The data obtained with a homologous series of di‐alkyl viologens are analyzed quantitatively using a kinetic model which includes the partition coefficients of the viologens and their radicals. For the most part, the reactivities of the viologen species are proportional to their concentrations in the membrane phase, but the locations and mobilities of the viologens in the membrane phase also have to be considered. In order for the Chl and viologen radicals to separate efficiently. the viologen radical has to be able to diffuse from the membrane to the water phase at a rate that competes effectively with reverse electron transfer within the radical pair complex (> 105 s−1).

In Situ Characterization of Redox Properties of Water‐Soluble Porphyrins Irreversibly Adsorbed on Gold Electrode Using the Electroreflectance Technique

The visible electroreflectance (ER) technique coupled to capacitive and voltammetric measurements has been used to characterize the redox properties of water‐soluble porphyrins [the meso‐tetrakis (4‐N‐methylpyridyl) porphine tetrachloride (TMPyP) and the meso‐tetrakis (4‐N‐methylpyridyl) porphine cobalt (III) pentachloride (Co(III)TMPyP)] in the potential range where they are irreversibly adsorbed on Au electrodes. The spectral features are analyzed in terms of reflectivity difference between the oxidized and reduced species and compared with optical properties of the bulk species. The ER spectra give direct evidence for the location of a reversible electron transfer reaction and allow us to determine the redox potentials of the Co(III)/Co(II) and Co(II)/Co(I) systems in their adsorbed state. The Co(III)/Co(II) couple is detected in an acidic solution at , and the Co(II)/Co(I) system is determined in neutral solution at . These potentials do not differ from the values obtained for the unabsorbed metalloporphyrin. These results provide evidence for the ability of a gold electrode covered with adsorbed porphyrin to constitute a modified electrode with well‐defined redox properties.

NMR and kinetic characterization of the interaction between French bean plastocyanin and horse cytochrome c

French bean plastocyanin is shown by stopped-flow kinetics to oxidize horse cytochrome c with k (298 K, I = 0.10 M) = 5.1 · 10 6 M −1 · s −1. The activation parameters demonstrate a satisfactory isokinetic correlation with those previously reported for plastocyanin-cytochrome f reactions. NMR line broadening and shifts of the hyperfine shifted resonances of cytochrome c(III) reveal that strong 1:1 complexes are formed with plastocyanin. The negative patch of plastocyanin and the heme edge region of cytochrome c are shown to be the interacting sites by the hyperfine shift perturbations and competitive binding experiments with Gd 3+, which associates selectively with the negative patch of plastocyanin. Complexation of plastocyanin and cytochrome c causes a small change in the heme electronic structure, but there is no NMR or optical evidence for significant conformational changes at either metal center. The rate of the reverse electron-transfer reaction within the plastocyanin-cytochrome c complex has been directly measured by NMR line broadening ( k rev (298 K) = 87 s −1). A rate for the intracomplex electron-transfer reaction ( k f (298 K) = 4.8 · 10 3 s −1) has been calculated from k rev and the optically measured equilibrium constant.

Novel Effect of Man‐Made Molecular Assemblies on Photoinduced Charge Separation. 4. Synthetic Bilayer Membrane of Amphipathic Viologen as an Efficient Electron Collector in Photosensitized Redox Reactions

AbstractAddition of synthetic bilayer membrane formed by a new type of amphipathic viologen containing two hexadecyl groups to aqueous solution of zwitterionic propylviologen sulfonate (PS2 – V2+) remarkably increased the yield of viologen photoreduction as sensitized by tris(2,2'‐bipyridine)‐ruthenium complex, Ru(bpy), in the presence of triethanolamine. Zwitterionic viologen was confirmed to shuttle as an efficient electron mediator between bulk aqueous solution and the interface of the bilayer membrane with a viologen head group. The quantum yield of viologen cation radicals was close to the net cage‐escape yield of photogenerated Ru(bpy) ‐ PS2‐V+ ion pair. The electrostatic field of positively charged interface of bilayer membrane was concluded to attract PS2‐Vc which transports electron to viologen head groups of bilayer membranes. Remarkable (500‐fold) suppression of the reverse electron transfer reaction was attained by the addition of viologen bilayer membrane. It was attributed to Coulombic repulsion between Ru(bpy) and positively charged interface of bilayer membranes. Relevance of the present system to chemical conversion of solar energy is discussed.

Electron-transfer kinetics and mechanism of di-imine bond formation in tetracyano(ethylenediamine)ferrate(II)

The oxidative dehydrogenation of the ethylenediamine (en) ligand in the complex [Fe(CN)4(en)]2– has been investigated by cyclic voltammetry, spectroelectrochemistry, and by stopped-flow kinetics with the [Fe(CN)6]3– ion, at 25 °C, and I= 0.100 mol dm–3(KCl). The first step in the oxidation process is assigned to a reversible electron-transfer reaction in the [Fe(CN)4(en)]2– complex, with E½= 0.303 V versus a normal hydrogen electrode. The oxidized ion undergoes successive electron transfer in the presence of OH– ions (pH > 11), forming the red FeII–di-imine complex. The observed rate constant is given by kobs.=(1.9 × 104)[Fe(CN)63–][OH–]. The di-imine complex is further oxidized in the presence of [Fe(CN)6]3– ions, with kobs.=(4.7 × 103)[Fe(CN)63–][OH–]. A mechanism is proposed involving FeIII intermediate species which undergo induced electron transfer assisted by OH– ions.